Origin of suspected black hole in Omega Centauri

Omega Centauri is suspected of being centred on a black hole and of perhaps being the remnant of a small galaxy rather than just a globular cluster. Could someone tell me, in either case, what the the accepted view is of how the black hole got there? Was it created as the star cluster formed? By gravitational collapse? Or did it seed cluster creation somehow? Why should black holes be found at the centres of galaxies? Did dark matter play a role in decorating galaxies and possibly star clusters with central black holes?

According to Dr Robert Massy, of the Royal Astronomical Society, the results (of researchers who confirm the existence of a black hole at the Milky Way centre) suggest that galaxies form around giant black holes in the way that a pearl forms around grit...... Dr Massy said: "Although we think of black holes as somehow threatening, in the sense that if you get too close to one you are in trouble, they may have had a role in helping galaxies to form - not just our own, but all galaxies"

For about three thousand centuries after the Big Bang, dark matter started collapsing into spongy clouds. As the universe cooled, protons and electrons eventually combined to form hydrogen (and some helium and lithium). These elements collapsed by the gravity of the already formed DM clouds and became stars. As with stellar nurseries today, some huge stars formed and after a few million years these stars went supernova. Some of the supernova remnants were black holes. Over the passage of time, the clusters of stars continued to collapse and through mergers ever more massive black holes formed. The gravity of the largest BH’s formed ever larger clusters. So why didn’t all the matter just fall into the BH? When a BH reaches a certain mass, more matter is “falling in” than can pass into the BH and the “run off” forms two opposing jets of energy thousands of light years long. When these objects were first observed they were titled Quasi-Stellar objects, or Quasars. Now the objects are called Active Galactic Nuclei or AGN. One of the side effects of this “quasaring” is that a shock wave forms and drives matter away from the BH. So the BH “switches off” and a galaxy of stars forms around it attracted by its gravity and held in place by the DM cloud. This is the most plausible theory.

Thanks for both the link and the clear summary of how people think galaxies come to be as they are. The seeming ubiquity of black holes in galaxies --- and perhaps even in smaller stellar structures --- is fascinating. Have you any links to more technical histories, say on the Arxiv? If so, I'd appreciate them.

stevebd1: thanks for this link, but my link is too slow to watch anything. If you have any text-type links I'd also like to follow them.

While Horizon is above average when compared to the average pop science programme, it still has it's fair share of filler so you might want to be selective when reading the text but there is useful information in there.

Trust the Beeb to come up with an excellent program. Thanks for this link, Steve, it's really useful and informative -- and quick to download on GPRS.

I notice that the program was broadcast some eight years ago, before really convincing evidence for dark non-baryonic matter via gravitational lensing in, say, the Bullet cluster was available, and before the WMAP results showed that most of the matter in the universe is indeed dark stuff. Unsurprisingly, there was then no pressing need to consider how dark matter collapses.

Do you know whether central black holes are thought (initially) to be made of non-baryonic stuff, or not, and if not -- why not? It seems to me than unlike the collapse of ordinary matter that interacts through all known channels (electroweak, strong and gravity), there is not much that can 'terminate' the infall of dark matter, except the formation of an event horizon. Has this sort of argument been considered?

I notice also that Arch2008 above refers to dark matter 'collapsing into spongy clouds', so the collapse has indeed been modeled, and is an important part of structure formation.

A lot of the hard data pieces to this puzzle have only been collected in the last couple of years or less........

Now I have an up-to-date set of references I can spend some time digesting. Thanks very much for posting them --- they're just the sort of thing I was looking for.

Perhaps you'd also care to comment on the possible constitution of galaxy-centre black holes? Do they have to form out of early-universe massive and short-lived baryonic-matter stars, or could they also form directly from collapsing clouds of dark matter? I notice that in the Oliveira et al. 1998 paper where the spherical collapse of both dark and baryonic matter were modelled hydrodynamically it was concluded that:

Sandra R. Oliveira said:

In the collapse of the DM the cooling-heating mechanisms and the photon drag, which are very important for the collapse of the baryonic matter, cannot directly alter the collapse of this component and thus the non-baryonic matter is rapidly concentrated in the innermost region of the clouds

Do you know whether central black holes are thought (initially) to be made of non-baryonic stuff, or not, and if not -- why not? It seems to me than unlike the collapse of ordinary matter that interacts through all known channels (electroweak, strong and gravity), there is not much that can 'terminate' the infall of dark matter, except the formation of an event horizon. Has this sort of argument been considered?

One popular candidate for dark matter is the neutralino which, due to being supersymmetric, is it's own anti-particle meaning if it came into too close contact with itself, it would annihilate, producing high-energy gamma rays (this is one process currently being used to detect dark matter). It's believed there is a concentration of neutralinos at the galactic (and solar) centre. While I think it's accepted that dark matter contributes to a black holes mass (as do photons and CMB), I think dark matter on its own would annihilate before collapsing into a black hole.

'An increased density of neutralinos may exist in the vicinity of the Galactic-centre or the Sun. This could have arisen in the initial formation of these objects. Also, neutalinos entering the solar system (or the Galaxy) may lose energy via elastic scattering with ordinary matter and become gravitationally trapped. Because of the capture and repeated scatterings, there would be a near-solar (or Galactic-centre) enhancement in the neutralino density. Such a local neutralino build-up may provide a detectable flux of annihilation products.'
Source- "scipp.ucsc.edu/milagro/papers/PhysRevD_70_083516.pdf"[/URL] Page 2

Yes, as you say, if dark matter does indeed turn out to be neutralinos with the expected supersymmetric properties, neutralino-neutralino annihilation could indeed rule out black holes forming from the collapse of dark matter.

But the density of a BH is inversely proportional to the square of its mass. So, if collapsing dark-matter clouds were massive enough, annihilation might be avoided by infalling neutralinos, if that's what dark matter is. The bigger the BH, the easier it forms.

Supposedly, the Black Hole's at the center of galaxies are the end result of countless mergers. Dark Matter, like anything else, could become a resident of the singularity. However, as John Wheeler posed, only mass, charge and angular momentum can be discerned from any BH. The information of the contents of the BH will only return as Hawking Radiation. However, it is possible that anything entering a BH becomes “frozen in time”, and thus a record remains plastered on the event horizon of everything that ever met this fate. So at least a theoretical possibility exists to decipher how much DM content is in this enigma.

But the density of a BH is inversely proportional to the square of its mass. So, if collapsing dark-matter clouds were massive enough, annihilation might be avoided by infalling neutralinos, if that's what dark matter is. The bigger the BH, the easier it forms.

You can make a stab at how big a cloud of dark matter would have to be in order to collapse into a black hole, based on DM being neutralinos and the DM density* of our galaxy being the critical density before neutralinos begin to annihilate each other (~3.739e-23 kg/m^3, which based on the neutralino being about 1 TeV, works out at about 21 neutralinos per m^3, which still seems quite high, though the neutralino might be anything up to 10 TeV) using the equation put together by George Jones in https://www.physicsforums.com/showthread.php?p=1406792#post1406792"-

Based on the parameters above, the answer comes out at about 1.396e+51 kg. Based on the supposed density of dark matter in our galaxy, this provides a volume of 3.734e+73 m^3 which is equivalent to a sphere with a radius of 219 million Lys.

Two orders of magnitude too big to account for supermassive BH's in galax centres!

Must one then conclude that the universe is littered with supermassive BH's that are created by an evolutionary process of gravitational collapse, accretion and mergers --- a process where only 15% of the universe's gravitating matter plays the central role? Remember that the collapse of baryonic matter is impeded and delayed by an intermediate chrysalitic stage of stellar existence, while the gravitational collapse of the remaining 85% dark mass fraction is free-fall, not so impeded.

It seems very odd to me that the fate of these two mass components should be so disparate --- baryonic matter forming BH's, dark matter forming diffuse haloes and spongy clouds.

Perhaps, as you seem to imply, Steve, the strange self-annihilating nature of neutralino dark matter is the key to this puzzle.

Do you know whether central black holes are thought (initially) to be made of non-baryonic stuff, or not, and if not -- why not? It seems to me than unlike the collapse of ordinary matter that interacts through all known channels (electroweak, strong and gravity), there is not much that can 'terminate' the infall of dark matter, except the formation of an event horizon. Has this sort of argument been considered?

Just thinking "out loud". Is it possible to turn this argument around?

If collapse of a clump of dark matter is asymmetric enough, parts of the clump will miss other parts, i.e., they will stream by each other. Couldn't "friction" (not sure if this is the right term) play a role in focusing an slightly asymmetrical collapse of baryonic matter, so that a black hole does form? Could "friction" play the same role in the accretion of stuff onto already formed black holes? Does this make any sense??

Could "friction" play the same role in the accretion of stuff onto already formed black holes? Does this make any sense??

The link you gave is very helpful. Thanks. And your comment about "friction" is, I think, germane to the peculiarities of dark matter collapse that have puzzled me. But I can't tell you how much sense it makes! I don't even know whether the following comments are relevant (or correct).

Cosmological dark matter is thought to be stuff that interacts with itself and other matter only via gravity. This is why, I think, in the link you gave, Peirani and Pacheko say that "dark matter particles constitute a collisionless fluid". They then add: "and, consequently, the accretion process should be less efficient than that expected for a dissipative fluid". Finally they conclude, in agreement with this statement that "dark matter contributes to no more than ∼ 10% of the total accreted matter" (of an SMBH).

The peculiarity that still puzzles me is why they assume that: "the accretion process (for dark matter) should be less efficient than that expected for a dissipative fluid (baryonic matter)" . It seems that "friction" (perhaps of the kind you were thinking of?), which I would have expected to slow processes down, instead seems to allow them to proceed faster.

I can only speculate that this must have to do with the slowness of virialising a collapsing structure in the absence of dissipative mechanisms like radiation emission. This may be why dark matter collapse, which P & P say is like adiabatic infall but with a modified adiabatic constant, is slow by comparison with baryonic matter collapse, but I don't really understand exactly why this should be so. I had fixed in my mind that dark matter would be like free-fall collapse --- not impeded by the formation of virialised structures like hot gas clouds and ultimately stars -- and therefore faster than baryonic infall. I guess the opposite is true -- the infall process is a lot more complicated than I thought.

Thanks. It's interesting to find that strange structures partly made of dark matter, like large 'fluffy' dark stars and fast-forming BHs, proposed by Paulo Gondolo, are being considered.

Since dark matter is thought to make up about 85% of all matter in the universe --- a major fraction, indeed --- surely one should guess that its gravitational collapse must somehow create a variety of structures other that 'spongy clouds' and 'dark haloes'. These were the only ones I'd seen mentioned until I followed the link you gave.

Other questions that still puzzle me are: What kind of dissipative mechanisms could facilitate collapse in a very nearly uniform collisionless gas of particles that is the dark matter of the early universe? Or: How can the virial theorem permit pure dark matter to collapse at all?

Other questions that still puzzle me are: What kind of dissipative mechanisms could facilitate collapse in a very nearly uniform collisionless gas of particles that is the dark matter of the early universe? Or: How can the virial theorem permit pure dark matter to collapse at all?

I think this is caused by self-gravity and slight fluctuations in density. See the links given in

I think this is caused by self-gravity and slight fluctuations in density.....

Thanks, George. I've used this link to Wolram's thread 'Sky map and dark energy', and have also read your #10 post there about the details of the growth of overdense regions. I've also been reading a recent paper by http://arxiv.org/abs/0810.2802" [Broken] about ' Pathways to massive black holes and compact star clusters in pre-galactic dark matter haloes with virial temperatures > 10000K'. which is relevant to the formation of SMBHs.

My remaining difficulty is of a more general nature, though.

It is this: for any structure to form by gravitational collapse and then endure in a collapsed state (rather than to eternally oscillate between its original configuration and its compact confuguration) mass/energy must be dissipated or removed from the structure. Examples: infall and capture in a closed orbit by a star of a object approaching along a hyperbolic path ; return of the Apollo missions to Earth; the birth of a star from a collapsing gas cloud (allowed by radiative dissipation), etc. etc.

I have so far been unable to find an description of the mechanism by which mass/energy is similarly removed form collapsing non-baryonic dark matter in order to stabilise an increasingly overdense region. It's probably so obvious a mechanism to workers in the field that they don't bother to mention it, just as people don't bother to mention that the emission of radiation is the crucial mechanism that allows a stable star to form. It's so obvious.

If one imposes suitably periodic boundary conditions on the (possibly infinite) universe to divide it up into imaginably finite pieces this question is accentuated, since across such boundaries one expects the net flux of mass/energy to be zero --- unless the universe is expanding (which it is!).

So perhaps in the end structure formation is only possible in an expanding universe??